1. Field of the Invention
This invention relates to a non-invasive device and method for monitoring concentration levels of blood constituents in living subjects such as humans or animals, using the near infrared portion of the light spectrum and, in particular, relates to a device and method suitable for continuously, or on demand, monitoring concentration levels of blood constituents within the human body.
2. Description of the Prior Art
Previous devices for non-invasively monitoring concentrations of blood constituents of a patient are known. Usually, a sensor is used to externally measure either the concentration of the constituent in gases emitted by the body; the concentration contained in perspiration; or the concentration contained in body fluids such as tears, saliva, or urine samples; or, alternatively, the blood constituent is measured using radiation passed through a part of the patient's body such as the earlobe. However, of the previous radiation devices, some have a radiation source which emits light in one wavelength only and are therefore not accurate or broadly applicable enough for practical use. Other previous devices have more than one light source but have only a limited number (three) of measuring wavelengths. Some of these previous devices have had a number of discrete wavelength sources obtained through use of a broad-band lamp whose light is optically coupled through a number of light filters, each with its own designated transmission wavelength, to the test sample. Some of these previous devices must measure both the intensity changes in the range of transmission wavelength and the changes in intensity distribution. Further, some previous devices are controlled to take a series of measurements at successively higher or lower wavelengths. This can be extremely time consuming. Further, some previous devices do not take into account changes in the thickness of one patient's earlobes compared to that of other patients or the change in size of a patient's earlobes or the change in the transmission path length due to the pulsing of blood through the patient; or, they do not take into account temperature variations in the earlobes from patient to patient; or, the results fluctuate with prolonged operation. Previous non-invasive devices are not sufficiently accurate to be used in place of invasive techniques in the measurement of blood constituent concentration levels by patients; or, they are designed to measure for one component only and must be physically changed to measure for a different component; or, the device takes an unreasonably long time to produce a result; or, they cannot produce results in an easy-to-use form; or, they cannot measure the results of two or more constituents simultaneously. Obviously, if the device gives an inaccurate reading, disastrous results could occur for the patient using the device to calculate, for example, dosages for insulin administration.
Invasive techniques of measuring blood constituents are, of course, in common usage. These techniques are painful, potentially dangerous and expensive to operate. The normal procedure is to obtain a blood sample from a vein and this sample is then tested in a medical laboratory, using a number of chemical procedures to measure each constituent separately. Alternatively, home glucose testing uses a finger puncture that is spotted onto an enzyme-based semi-permeable membrane test strip and is allowed to react for a certain length of time, with insulin administration then based upon either a visual colour comparison with a standard colour chart or by means of a more accurate and unambiguous spectroscopic technique (for example reflectance). There is a risk of infection and sometimes a patient can develop a rash when these invasive techniques are used.